1. Field
This disclosure relates to color copiers, more particularly to a method for adjusting background colors in color copiers.
2. Background
Color reproduction devices, such as copiers and fax machines, typically have a processing pipeline that converts a scanned image into data, and then processes the data into the appropriate format for printing. For example, a user places a color original on the platen of a color copier or into the tray of a fax machine. The device scans the document using a set of red, green and blue (RGB) sensors and produces a data representation in terms of the RGB signals at each location on a pixel grid. In many applications, the RGB data is then converted to a luminance/chrominance color space because this space makes it possible to separately process chromatic and intensity information. Additionally, it is possible to represent the chromatic and intensity information at different resolutions, thus the image in the luminance-chrominance color space can be represented more compactly than in an RGB color space.
Luminance-chrominance color spaces typically have three components. One example of this type of color space is CIE Lab. “L” represents the dark-to-light lightness dimension. “a” and “b” represent the chrominance components. Although CIE LAB is also designed so that equal distances between points in the space are approximately equal in terms of perceived color difference, this feature is not central to the problem with which we are concerned. Other luminance-chrominance color spaces can be used where this property is not present. Technically, CIE L values are lightness and the exact definition is different than the definition of luminance. Either definition could be used to code the intensity information at a pixel. The intensity dimension will be referred to here as lightness or L values.
In some cases, such as if the paper upon which the original image resides were not white, the background pixels of the LAB image would be printed as colors. This takes a longer amount of time and wastes toner or ink to render the background color, which was more than likely not desired. Most users that are copying from a colored piece of paper do not want the color reproduced. Typically, if users want a copy that is on colored paper, the user will load color paper into the copier before copying the original.
Currently, most copiers employ background suppression to eliminate the background paper color. Most perform this suppression in the lightness component only. The lightness of the background pixels are typically mapped to white, setting their lightness values to the corresponding white level for that copier. In most copiers, lightness will be set to white at 255, assuming 8-bit processing where values range from 0 to 255.
Once the LAB space is adjusted to suppress the background, the LAB data is mapped to the appropriate color space for that print engine. These are typically a cyan-magenta-yellow (CMY) space or a cyan-magenta-yellow-black (CMYK) space.
However, a problem occurs if a portion of the original has highlighting, where a lighter color is used to set off a darker object. Other examples exist, but the most common type is where text is highlighted with a light yellow, pink or blue highlight surrounding the text characters. Highlight colors are also frequently used in maps. With the usual method of color processing, the lightness of these pixels surrounding the text characters is mapped to white and the highlighting is not preserved in the color copy, which may have been desired.
Therefore, it would be useful to have a method of background suppression that allows preservation of these highlighted colors, while avoiding the unnecessary extra processing and toner/ink use that would occur with colored paper.